The Grand Slam (Earth Quake bomb), was a very large freefall bomb developed by the British aeronautical engineer Barnes Wallis (who also made the bouncing bomb) in late 1944. At a weight of 9.98 t (22,000 lb) the Grand Slam was almost twice the weight of his previous large bomb, the 5.44 t (12,000 lb) Tallboy. Both weapons were intended for use against large and protected buildings, structures against which smaller bombs would be ineffective. The idea of the earthquake bomb was explored by Barnes Wallis at the very start of World War II but at the time there were no aircraft capable of carrying the 10-ton weapon he envisaged (notwithstanding Wallis's suggestions to build one, a six-engine high-altitude bomber called "Victory"). Wallis returned to his designs in the latter part of the war and the first earthquake bomb he developed was the 5-ton Tallboy. It proved effective in demolishing large structures, including heavily-protected bunkers (it was thus an early "bunker buster").

The reasoning behind the earth penetration bomb is that explosive energy is transmitted more efficiently in a non-compressible medium. Barnes Wallis used the non-compressible nature of water as a factor in the bouncing bombs. The earthquake bomb was designed to penetrate the earth and explode some 30 m down. An explosion carried through the medium of the earth would thus cause damage for a much greater distance than if the bomb were to explode in open air. Wallis also understood that bomb aiming was very poor at the time. The major advantage of the earthquake bomb, therefore, was that it could miss by hundreds of yards and still achieve the desired result. The intention before the war started was to destroy dams, railway bridges and general infrastructure. Thus it is possible that German industry and infrastructure could have been seriously damaged with minimal loss of civilian lives, compared to area bombing. His ideas were not fully understood, appreciated, or even realisable at the time.

The design was highly aerodynamic, with a long tail incorporating offset fins, causing it to spin as it fell and stabilizing it, due to the gyroscopic effect, much as the spin imparted by the rifling of a gun barrel increases the accuracy of a bullet. The spin also allowed the bomb to reach supersonic speeds, as the increased stability enabled it to pass through the sound barrier without wobbling or being thrown off-course.

The Grand Slam had a much thicker case than typical World War II bombs, so it would survive the impact of hitting a hardened surface. The hardened steel bomb casing was cast in one piece in a sand mould using a concrete core. Torpex was then poured in, bucket by bucket. When filled, the hot molten explosive took a month to cool down and set, greatly limiting production. Like the Tallboy, the rate of production and material and manpower investment in each bomb meant that aircrews were told to land with their unused bombs on board, rather than jettison them into the sea if a sortie was aborted.

When dropped from high altitude onto compacted earth, the Grand Slam would penetrate over 40 metres into the ground. The explosion would leave a camouflet (cavern) which would undermine foundations of structures above, causing collapse. This is what happened to the Bielefeld railway viaduct, the first enemy target destroyed by a Grand Slam.

The 'B1 (Special)' Lancaster bomber could only carry one at a time and it had to be dropped from 22,000 feet (6700 m) which limited its accuracy. The Grand Slam was first used on March 14, 1945 when the Royal Air Force No. 617 "Dambusters" Squadron, lead by Squadron Leader C.C. Calder, attacked the Bielefeld railway viaduct destroying two spans of the viaduct.[1]

The viaduct at Arnsberg was bombed on 15 March 1945 with 2 Grand Slams and 14 Tallboy bombs but they failed to bring the viaduct down. Four days later on 19 March 1945 another attack by No 617 Squadron using 6 Grand Slams was successful and a 12 m (40 ft) gap was blown in the viaduct. Farge is a small port on the Weser River north of Bremen, and was the site of an oil-storage depot and the Valentin submarine pens that were attacked by the RAF on 27 March 1945. The pens had a ferrous concrete roof up to 7 metres (23 feet) thick. Two Grand Slam bombs penetrated parts of the pen with a 4.5 m-thick roof.

Grand Slams were also successfully used against the Huuge and Brest submarine pens. By the end of the war 41 Grand Slam bombs had been dropped, mainly against bridges and viaducts.